1. Field of the Invention
The present invention relates to a gearshift control device for a chain and sprocket transmission system.
2. Description of Background and Relevant Information
In the field of bicycles, transmission is ensured, as a general rule, by an endless chain that is returned by a front transmission device and a rear transmission device.
The front transmission device includes a set of toothed chainwheels having varying diameters, which are rotationally coupled to the pedal cranks. Similarly, the rear device includes a set of sprockets, the number of teeth of which varies progressively. The sprockets are coupled to the rear wheel hub. The numbers of chainwheel and sprocket teeth used by the chain determine the transmission ratio between the crankset and the wheel. A front derailleur and a rear derailleur make it possible to force the chain into changing the chainwheel or the sprocket in order to vary the transmission ratio.
The rear derailleur usually includes one or two chain pulleys, on which the slack side of the chain passes. These pulleys are supported by an articulated cage generally of the deformable parallelogram type, which enables the displacement of the pulleys along a directional component that is parallel to the axis of the sprockets in order to position them in the plane of a defined sprocket.
At the front, the derailleur usually includes a movable fork-shaped cage that houses the tensioned side of the chain.
These front and rear derailleurs are mechanically controlled by cables connected to control levers or handles that are located on the handlebar or the front of the frame, within reach of the cyclist's hand. For these derailleurs, all the force used for gear shifting is provided by the cyclist's manual actuation of the control levers or handles.
Motorized derailleurs also exist, which generally implement an electric motor driven by an electric control. Such a derailleur is disclosed, for example, in U.S. Pat. No. 3,919,891.
Another electric derailleur construction method is proposed in European Patent Publication No. 0 558 425 and U.S. Pat. No. 5,358,451. Instead of being furnished with an electric motor, the force enabling the displacement of the movable assembly is provided by the rotation of one of the derailleur pulleys that are driven by the chain.
According to the construction disclosed, the derailleur includes a fixed assembly mounted on the frame and a movable assembly bearing the chain pulleys. The movable assembly is slidably mounted along an arm for supporting the fixed element; and an indexing device defines stable positions of the movable assembly in correspondence with each of the sprockets. The rotational movement of one of the chain pulleys is transformed by the guiding path of a cam into a to-and-fro movement of a rack housed within the support arm. During gearshift control, a solid connection is momentarily established between the rack and the movable assembly, in order to initiate the displacement of this movable assembly, along with the rack, toward an adjacent stable position defined by the indexing device.
This device yields good results. However, the mode of connection between the fixed assembly and the movable assembly by a sliding principle imposes constraints that result in a relatively cumbersome final construction and sealing conditions that are difficult to control.
Furthermore, completely mechanical gearshift systems are known, in which a portion of the mechanical force required to control the derailleur is drawn from the crank shaft. The documents EP 0 482 559 and U.S. Pat. No. 5,201,236 generally disclose a front derailleur equipped with such a system shown in FIG. 16 and subsequent figures. In addition, each of the following documents discloses a gearshift control device equipped with an assisting apparatus, although these devices involve a relatively complicated construction: EP 1 378 436 and family member U.S. Pat. No. 6,848,335; EP 1 394 034 and family members U.S. Pat. No. 6,868,752, US 2004/0005949, US 2004/0025620; EP 1 394 035 and family member U.S. Pat. No. 6,899,649; and EP 1 394 036 and family member U.S. Pat. No. 6,835,148.
Under these circumstances, and in view of these aforementioned disclosures, there is a need for a gearshift control device with improved construction in terms of space requirement and imperviousness to infiltration of outside elements.